Conventionally, a Micro Electro Mechanical Systems (MEMS) mirror which reciprocally oscillates is known as a reflecting mirror for deflecting scanning laser light toward a photoconductor (see, for example, Japanese Unexamined Patent Publication No. 2004-177543). In FIG. 3 of Japanese Unexamined Patent Publication No. 2004-177543, a MEMS mirror is denoted by 8 and piezoelectric bodies as drive sources for oscillating the MEMS mirror 8 are denoted by a, b, c and d. The drive sources a, b are driven in the same phase and the drive sources c, d are driven in a phase opposite to the drive sources a, b, whereby the MEMS mirror 8 undergoes torsional oscillation. In this way, scanning is performed by reflecting and deflecting laser light incident on the MEMS mirror 8.
Further, a block diagram of a drive circuit for driving the drive sources a, b, c and d is shown in FIG. 2 of Japanese Unexamined Patent Publication No. 2004-177543. In this FIG. 2 of Japanese Unexamined Patent Publication No. 2004-177543, a sine wave is generated in an oscillator 121a and that sine wave is input to a phase inversion circuit 121b and a phase shifter 121c. In the phase shifter 121c, a signal which is so adjusted that an image signal corresponds to the phase of the MEMS mirror 8 is generated. This signal is voltage amplified by an amplifier 121e and supplied to the drive sources a, b. Further, the sine wave generated in the oscillator 121a has the phase inverted through the phase inversion circuit 121c and is supplied to the drive sources c, d by way of the phase shifter 121d and an amplifier 121f. 
Further, Japanese Unexamined Patent Publication No. 2008-40460 discloses a technique for configuring a driving unit for a mirror as one electromagnetic actuator with a coil and a permanent magnet and oscillating a mirror by torque acting on the permanent mirror when a current flows in the coil. Also in this technique, after a sine wave is generated in an arbitrary waveform generator, that sine wave is amplified by an amplifier as in the above Japanese Unexamined Patent Publication No. 2004-177543.
Japanese Unexamined Patent Publication No. 2008-40460 also discloses a technique for correcting a displacement angle (deflection angle) of a mirror which changes due to an environmental change and a change over time. In this correction technique, a light receiving element for receiving scanning light deflected by the mirror is arranged at a predetermined position, an output of this light receiving element is taken in and the phase and amplitude of a sine wave generated in the arbitrary waveform generator are so adjusted that the scanning light passes through the light receiving element at a desired timing.
However, the oscillator capable of generating a sine wave and the amplifier capable of accurately amplifying a sine wave, which are disclosed in Japanese Unexamined Patent Publication No. 2004-177543 and Japanese Unexamined Patent Publication No. 2008-40460, are expensive. Particularly, the oscillator capable of generating a sine wave by changing a phase and an amplitude as disclosed in is even more expensive. Thus, there has been a room for improvement in terms of cost in configuring an optical scanning device and an image forming apparatus including a MEMS mirror and capable of correcting a drive voltage for driving the MEMS mirror to compensate for a change in the deflection angle of the MEMS mirror.